Variation Of Thermal Diffusivity Research Articles

Photoacoustic Investigation of Glass Transition in AsxTe1−x Glasses

Photoacoustic (PA) technique is used to study glass transition and temperature dependence of thermal diffusivity in AsxTe1-x glasses with 0.25 0.60. PA amplitude goes through a minimum and the phase shows a maximum at glass transition temperature Tg. The variation of thermal diffusivity with temperature shows sharp decrease near Tg. The variation of thermal diffusivity with composition shows maximum at = 0.40 for all temperatures T Tg.

A model for including thermal conduction in molecular dynamics simulations

A technique is introduced for including thermal conduction in molecular dynamics simulations for solids. We develop a model to allow energy flow between the computational cell and the bulk of the solid when periodic boundary conditions cannot be used. Thermal conduction is achieved by scaling the velocities of atoms in a transitional boundary layer. The scaling factor is obtained from the thermal diffusivity, and the results show good agreement with the solution for a continuous medium at long times. We have investigated the effects of different temperature and size of the system, and of variations in strength parameter, atomic mass, and thermal diffusivity. In all cases, no significant change in simulation results has been found.

Temperature dependence of thermal conductivities and thermal diffusivities of composites using transient hot-strip method

Simultaneous measurements of the thermal conductivity and thermal diffusivity of four composite building materials, namely black marble stone, normal building stone, red sand brick and cement brick have been made at room temperature using the transient hot-strip method. The power supplied to these samples is quite large compared with that for loose building materials owing to their composite (brick) nature. The temperature dependence of thermal conductivity and thermal diffusivity of these materials has also been investigated in a temperature range from 10 to 60°C. Temperature stability is assured within 0.1°C by using a platinum resistance thermometer in the chamber containing samples and a digital voltmeter to record any variation in resistance. The results of these experiments show that there is a very slight variation in thermal conductivity and thermal diffusivity of these materials in this temperature range.

Interstitial Air Pressure Dependence of Effective Thermal Conductivity and Diffusivity of Rajasthan Desert Sand Using Transient Hot-Strip Method

The transient hot strip (T H S) method used for the simultaneous measurement of effective thermal conductivity and diffusivity of loose granular two-phase materials at normal pressure and room temperature is now used for the measurement of effective thermal conductivity and diffusivity of Rajasthan Desert sand at different interstitial air pressure and room temperature. Experimental values of effective thermal conductivities and diffusivities at different interstitial air pressures have been compared with the estimated values using empirical models for the variation of thermal conductivity and diffusivity with interstitial air pressures. Estimated values are in good agreement with the experimental results in the interstitial air pressure range from 1 mm to 760 mm of mercury, whereas in the low pressure range (0.1 mm to 1 mm Hg) deviations from the experimental results have been observed.

Diurnal Thermal Regime in a Peat-Covered Palsa, Toolik Lake, Alaska

Strong spatial and temporal variations in temperature and effective thermal diffusivity are apparent from short-term thermal records measured in the peat atop an ice-cored palsa in northern Alaska. Intersite differences in near-surface temperature regimes apparently result from topographic influences, vegetative cover, and site wetness. Values of effective thermal diffusivity indicate that ablation of the ice core is inhibited by advection of cold water near the ice-peat contact and by internal evaporation near the surface. These findings support the conclusions of earlier investigators, who emphasized the importance of peat for maintaining palsas near their equatorward limit. Key words: active layer, frost mound, palsa, peat, permafrost, soil temperature, thermal properties, Alaska

Dependence of thermal diffusivity on organic content for Green River oil shales—Extension of the modified Cheng-Vachon model to the parallel heat-flow case

In an earlier paper [J. Appl. Phys. 50, 2776 (1979)], the modified Cheng-Vachon model was found to give good agreement with experimental data on the variation of thermal diffusivity with organic content for Green River oil shales. Calculations using the model were carried out for the case where heat flows in directions perpendicular to the shale stratigraphic planes. In the present paper, the above model is modified to account for experimental trends in the parallel heat-flow case. The modified model provides a self-consistent explanation for the lower degree of anisotropy (relative to theory) that has been experimentally observed for the thermal diffusivity of Green River oil shales.

Thermophysical properties of oil shale minerals

The thermal diffusivity values of eight minerals that are commonly associated with oil shales of the Green River formation have been measured by the laser flash technique. Data are presented in the temperature range 25–500°C for quartz, dolomite, calcite, plagioclase, analcite, pyrite, potassium feldspar and low albite. A comparison of the thermal conductivities of some of these minerals, calculated from experimentally measured thermal diffusivity and density, with the experimental values reported in the literature reveals good agreement. Trends in the variation of thermal diffusivity with temperature and anisotropic effects in thermophysical parameters are discussed from the point of view of grain boundary effects in these polycrystalline mineral aggregates.

A new apparatus to measure the thermal conductivity and diffusivity of thermoplastic and thermosetting polymers

AbstractA new apparatus has been constructed to measure the thermal conductivity and the thermal diffusivity of reactive and nonreactive liquids and solids. The method used consists in suddenly subjecting the test specimen to a thermal flux and measuring the velocity of the heat wave across the sample. Maintaining the heat flux leads to a steady thermal gradient across the sample from which the thermal conductivity can be determined. The test duration is short relative to durations employed by existing conventional techniques and thus causes only a small thermal disturbance to the sample of the order of 1 to 2°C. Thermal properties of liquids and solids, including thermoplastics and a typical epoxy resin during cure with different equivalents of an aromatic diamine hardener have been measured near room temperature. Experimental data compare favorably with literature values for similar systems. Variations in thermal conductivity and diffusivity during cure have been obtained through a series of individual discrete measurements to cover the whole isothermal cure history. The results have been analyzed and discussed with respect to the cure characteristics of the resin and the nature of the physicochemical changes the material undergoes during cure.

Estimation of Wintertime Soil Temperatures1

Winter soil temperatures are important to developmental processes of insects, yet are inconvenient to record at various depths. Therefore a method of estimating these temperatures would be useful in field applications. Heat transfer equations are not suited for predicting soil temperatures at various depths because of the variable thermal diffusivity coefficient found in cultivated fields. However, simple linear regression analyses provide good estimates of mean soil temperatures at depths of 0, 3, 6, 9, and 12 in. by using daily mean air temperature as the independent variable. Even better estimates can be obtained at subsequent depths by using soil-surface temperature as the independent variable in the linear regression equation.

Static and recovery characteristics of wall-stabilized d.c. arcs in nitrogen

Measurements are given of the field gradient characteristics for arcs operating in tubes of different diameters in nitrogen. The recoveries of conductance for these tubes (diameters 4, 5 and 7 mm) are also given over a period of 300 μs following rapid interruption (1 μs). The arc behaviour under imposed gas flow conditions of 0-2·5 1. min−1 is also described. In particular it is found that the recovery, when presented as a log conductance against time plot, has three linear regions. Time constants derived from these linear portions do not seem to exhibit a simple dependence on the confining tube radius and are at variance with previous theories.A solution of the energy balance equation is also given, for conditions of variable thermal diffusivity combined with a non-linear relationship between the electrical conductivity and heat potential functions, and this gives close agreement with the experimental data.

1011 高温時における建築材料の温度伝導率及び熱伝導率の変化 : (測定装置及び方法)

1011 高温時における建築材料の温度伝導率及び熱伝導率の変化 : (測定装置及び方法)

Some Thermal Characteristics of Porous Rocks

Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 375–378. Abstract Results of experimental measurements of heat capacities and thermal conductivities of some typical porous rocks are presented. Measured heat capacities agree closely with values calculated from known chemical compositions of the rocks. On the basis of this agreement, heat capacities of fluid-saturated rocks were calculated. Thermal conductivities of the rock samples were measured under various conditions of fluid saturation. From these data thermal diffusivities were calculated. A significant variation of thermal diffusivity with temperature is indicated. Introduction Knowledge of the thermal characteristics of fluid-bearing porous rocks has become increasingly important with the development of thermal oil-recovery processes. Reliable thermal data for petroleum reservoir rocks are not available and approximate values are generally assumed for reservoir calculations. The effects of temperature, pressure and fluid saturation on thermal properties of porous rocks have not been carefully studied. The purpose of the present work was to measure and report the thermal characteristics of some typical porous rocks. A group of eight sedimentary rock samples, including limestone, shale, sandstone and siltstone, was selected for the study. Properties reported include heat capacities, thermal conductivities and thermal diffusivities. The effect of fluid saturation on these properties was also considered. Determinations of thermal properties require precise measurements which are time consuming and difficult to reduce to a routine basis. Methods have not been developed for measurement of thermal properties under conditions of pressure, temperature, and fluid saturation, which may be encountered in thermal recovery processes. As one approach to these problems, some preliminary work on methods of estimating thermal properties from other more readily measurable characteristics of the rock-fluid system is reported.

Measuring the Thermal Diffusivity of Metals at Elevated Temperatures

Abstract The Angström-bar method has been modified and adapted to the measurement of thermal diffusivity of metals for temperatures ranging from 300 to 1050 F. A small, sinusoidal temperature fluctuation is superimposed on the mean temperature of a thin-wall tubular specimen by means of simultaneous heating and cooling at one end. Thermal diffusivity was calculated from the phase shift and amplitude ratio of the temperature waves determined at two measuring stations. The effect of possible surface heat loss is considered. Measurements were made on AISI 1018, 1045, 3140 steels, and 1.1 per cent carbon tool steel. Results obtained for electrolytic tough-pitch copper and 2S aluminum showed good agreement with a recent theory of Storm concerning the variation of thermal diffusivity of simple metals with temperature.

Stability and accuracy of numerical solutions of the heat flow equation

The merits of different finite difference representations of the heat flow equation and a convection boundary condition equation are discussed in terms of the stability and accuracy of the solutions they yield. A new method of deriving a condition for stability is given. This is applied to examples of heat conduction involving variable thermal diffusivity and heat transfer by convection at the surface. Some finite difference forms of this boundary condition impose stability criteria which are stricter than, or additional to, that required in the case of a prescribed surface temperature. A new finite difference representation of the surface heat flux equation is discussed.